The invention relates to a heated roller for a heating device for warming a film strip comprising thermoplastic plastic, wherein the roller has a tubular shape and is provided with holes or bores in its wall for inserting heating cartridges or providing passage for a heating medium.
German Patent No. DE 33 11 988 C1 discloses a heating device having heated rollers. It is known that these heated rollers comprise an aluminum alloy that is centrifugal-cast or chill-cast to form a tube having an outer diameter in the order of magnitude of 400 to 600 mm and a wall thickness of about 30 mm. A plurality of bores is cut into the circumference of the wall for mounting electrical heating cartridges. A temperature-control device is used to heat the heated rollers to a set temperature.
The surface of the heated rollers is preferably provided with a non-stick coating, such as Teflon®(polytetrafluoroethylene), to prevent the hot film from sticking, and to allow vapors formed by the film and dirt to be easily removed from the surface of the heated roller.
This type of heated-roller production is associated with numerous disadvantages. There are few manufacturers who are even willing or able to cast heated rollers in this diameter range because of the associated production problems. Because the finished rollers must have a clean, pore-free surface, slag inclusion and resulting porous spots inevitably lead to rejected products or time-consuming touch-up work. These porous spots are, however, practically unavoidable, and cannot be seen until the surface of the heated roller is machined—typically turned.
Creating the bores for the heating cartridges is extremely cost-intensive because the heated rollers must have a width of up to about 900 mm, depending on the width of the film strip to be heated. The bores must be relatively identical with respect to tolerance. A minimum size must be assured. Otherwise, the heating cartridges cannot be inserted. On the other hand, if the bores are too large, the heat transfer is non-uniform. The heated rollers then have an uneven temperature distribution. Such bores can only be produced with costly, special deep-hole boring machines. A risk associated with these machines is that the drill bit may travel beyond the defined length and puncture the thin roller wall. A different distance from the surface also results in a non-uniform temperature distribution.
The surface coating of the heated rollers is also problematic, because only a few manufacturers are capable of coating such large parts. Heated rollers must be handled carefully to avoid damage to the surface coating, which would stipulate a costly re-coating of the entire heated roller. This must be taken into consideration in the transport from the coating site to the user, during storage until installation, and during installation itself. Costly packaging is required between the coating process and the actual use. Suitable means of transport are also necessary. The large heated rollers also require a correspondingly large amount of storage space.
DE 35 39 500 A1 discloses a roller with which a film strip is heated in one segment, then cooled in a second segment. The roller comprises an outside ring and an inside ring, with webs between the two rings. The outside and inside rings are connected in one piece over the entire roller circumference. It can be assumed here, there is no description, that the parts are welded together. This roller does not permit the insertion of heating cartridges. It is also not possible to maintain a uniform roller temperature, with the use of a detector element, because the wall is too thin. The aforementioned coating-related problems also exist here, including the fact that it is impossible to exchange individual roller parts in the event of damage.